A very popular type of window covering for some time has been one referred to as a venetian blind in which a plurality of slats are adjustable to various relative angles to one another to control the amount of light passing through (and thus the degree of viewing) and which also can be raised or lowered to any desired height. A further highly successful window covering unit having some features in common with a venetian blind includes a pleated fabric which can be raised or lowered to any desired height.
Adjustment of the cords to produce a desired height of the window covering for either a conventional blind or a pleated fabric window covering is typically accomplished by manipulation of an extent of the cords hanging downwardly from a head rail via which the window covering is mounted and supported. After an adjustment of the window covering has been accomplished, it is necessary to secure the cord in order to maintain the window covering height at its new adjusted position.
A very early manner of achieving this was to provide a peg or post located immediately adjacent the blind to which the cord was tied in a suitable manner. This approach was not fully satisfactory in that frequently the cord would become loosened, allowing the blind to fall, and in many other circumstances there was simply no convenient place to which the tie post could be mounted. More recently, several different types of so-called "cord locks" have been provided incorporated into the head rail, which upon selective angular adjustment of the cord, enables vertical adjustment of the blind height, and on release of the cord at a new angle of adjustment the cord is automatically locked into the new position which, in turn, keeps the blind height constant until it is affirmatively changed.
One prior art cord lock, is that shown in U.S. Pat. No. 4,245,688 in which a fixed guide roller is mounted between front and rear plates and a lock roller is slidably related along arms of a slot within which it is received in order to releasably secure a cord between the two rollers.
A copending U.S. patent application Ser. No. 242,057, WINDOW BLIND CORD LOCK by C. M. McNeil and L. G. Valle, which application is assigned to the same assignee as the present application, discloses a venetian blind cord lock having a movable smooth surfaced roller which wedges against a fixed roller to secure the adjustment cord therebetween. Although the construction of this prior known cord lock represents a considerable step forward in regard to economy of manufacture, it still is now completely satisfactory in that on occasion, depending upon unusually large pulling forces and direction of pull applied to the cords, the moving roller becomes canted along its path of movement, resulting in slippage of the cord within the cord lock. In addition, in the event of wear of the cord to the point where substantial reduction in cord cross section is experienced, this can result in the cord lock being ineffective on that portion of the cord, such that the cord will slip and, of course, the blind will experience a corresponding amount of readjustment.